Current limiting power interruption requires a current interruption device that rapidly and effectively brings the current to a low or zero value upon the occurrence of a line fault or overload conditions.
Circuit protection devices protect electrical equipment from damage when excess current flows in the circuit due to overload or short circuit conditions. Such devices have a relatively low resistivity and, accordingly, high conductivity under normal current conditions of the circuit but are "tripped" or converted to high or complete resistivity when excessive current and/or temperature occurs. When the device is tripped, a reduced or zero current is allowed to pass in the circuit, thereby protecting the wires and load from electrical and thermal damage until the overload or fault is removed.
Conventional circuit protection or current limiting devices include, but are not limited to, circuit breakers, fuses, e.g., expulsion fuses, thermistors, e.g., PTC (Positive Temperature Coefficient) conductive polymer thermistors, and the like. These devices are current rated for the maximum current the device can carry without interruption under a load.
Circuit breakers typically contain a load sensing element, e.g., a bimetal, hot-wire, or magnetic element, and a switch which opens under overload or short circuit conditions. Most circuit breakers have to be reset manually at the breaker site or via a remote switch.
Fuses typically contain a load sensing fusible element, e.g., metal wire, which when exposed to current of fault magnitude rapidly melts and vaporizes through resistive heating (I.sup.2 R). Formation of an arc in the fuse, in series with the load, can introduce arc resistance into the circuit to reduce the peak let-through current to a value significantly lower than the fault current. Expulsion fuses may further contain gas-evolving or arc-quenching materials which rapidly quench the arc upon fusing to eliminate current conduction. Fuses generally are not reusable and must be replaced after overload or short circuit conditions because they are damaged inherently, when the circuit opens.
Various fusible elements, gas-evolving materials and fuses are shown for example in U.S. Pat. Nos. 2,526,448; 3,242,291; 3,582,586; 3,761,660; 3,925,745; 4,008,452; 4,035,755; 4,099,153; 4,166,266; 4,167,723; 4,179,677; 4,251,699; 4,307,368; 4,309,684; 4,319,212; 4,339,742; 4,340,790; 4,444,671; 4,520,337; 4,625,195; 4,638,283; 4,778,958; 4,808,963; 4,950,852; 4,952,900; 4,975,551; and, 4,995,886.
The resistance of a circuit element such as a fuse is a matter of its material and its dimensions. Resistance along the circuit path decreases with increasing cross-sectional area. Thus resistive heating of the circuit element, which is a function of current and resistance according to I.sup.2 R, is a function of current density. In a typical fuse, the fusible element has a small cross-sectional area along the direction of current flow, so as to concentrate heating at the fusible element, and comprises a low melting temperature material.
Thermistors are a particularly useful type of circuit protection devices that employ heating, especially positive temperature coefficient (PTC) conductive polymer thermistors. PTC conductive polymers typically comprise a polymer, e.g., a thermoplastic, thermoset, or elastomeric polymer, having conductive particles, e.g., carbon black, graphite, metal, or metal oxide, dispersed in the polymer matrix. PTC conductive polymers have low resistivity under normal current conditions, but due to the positive temperature coefficient of their resistance, undergo an exponential increase in resistivity as their temperature rises through resistive heating (I.sup.2 R) caused by fault current. The resistance becomes substantial over a particular current and/or temperature value which is referred to as the switching temperature or anomaly temperature. PTC conductive polymers can be placed in series with a load, thereby introducing increased resistance into the circuit to reduce the peak let through current to a value significantly lower than the fault current.
Once the fault current dissipates, the PTC conductive polymer material cools and reverts back to its original low resistivity. Accordingly the PTC conductive polymer is automatically resettable over a number of thermal cycles to provide a reusable circuit protection device. However, PTC conductive polymer devices are subject to degradation as a result of material resistivity changes over thermal cycles.
Various PTC conductive polymers and thermistors are shown for example in U.S. Pat. Nos. 2,952,761; 2,978,665; 3,243,753; 3,351,882; 3,571,777; 3,757,086; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,950,604; 4,017,715; 4,072,848; 4,085,286; 4,117,312; 4,177,376; 4,177,446; 4,188,276; 4,237,441; 4,242,573; 4,545,926; 4,647,894; 4,685,025; 4,724,417; 4,774,024; 4,775,778; 4,857,880; 4,910,389; 5,049,850; and, 5,195,013.
What is needed is an improved automatically resettable electrical circuit protection device with improved circuit interrupting capacity and longer life.